The present invention relates to the heat treatment of a coil for use as a superconducting coil, and in particular coils which are manufactured by a xe2x80x9cwind and reactxe2x80x9d process, for use in a superconducting magnet The present invention further relates to a coil for use as a superconducting coil which is suitable for undergoing such a heat treatment process.
The xe2x80x9cwind and reactxe2x80x9d process is a technique which is frequently used with superconducting materials which, in their fully reacted state, are so brittle that winding them into a coil would cause fracture. To avoid problems of this kind, the coils are first wound from the superconducting wire or coil material in an unreacted state, where the material is ductile. A reaction heat treatment is then applied to bring about the desired reactions in the superconducting material.
A commonly observed problem in xe2x80x9cwind and reactxe2x80x9d coils made from materials such as Bismuth Strontium Calcium Copper Oxide (B2212), is that the critical current of the superconducting coil material is substantially less in the coil than it would be if the same coil material was heat treated in the form of a short sample. This is caused by problems relating to the diffusion of heat, and the diffusion of oxygen, through the volume of the coil.
The problems associated with the diffusion of heat through the coil will now be outlined. FIG. 1 shows a typical reaction heat treatment cycle for the melt processing of B2212. This processing is carried out in an atmosphere of pure oxygen or an oxygen nitrogen mixture. During a first stage 1 of the process, the temperature is ramped up over a period of several hours to a level which is just sufficient to melt the B2212. As shown in more detail in FIG. 2, during a second stage 2, the temperature is maintained substantially at the melt temperature for a few minutes, before the temperature is ramped down during a third stage 3 by approximately 30xc2x0 C. A fourth stage 4 is a very slow ramp down which serves to anneal the material, after which the temperature is reduced rapidly to ambient during a fifth stage 5.
For optimum superconducting properties it is essential for this heat treatment to be tightly controlled. In particular, the peak melting temperature of approximately 885xc2x0 C. must be held to a precision of 1xc2x0 C. Furthermore, it is important for the melt time to be controlled at a precise time of a few minutes.
These conditions can readily be obtained for short samples of conductor in a furnace with a precisely controlled temperature profile. In coils however the temperature within the winding is not uniform because heat takes a finite time to diffuse from the outside of the coil to the inside.
FIG. 2 shows measurements of the temperature inside and outside a small experimental coil, during the first, second and third stages 1,2,3 of the melt sequence. It may be seen that the temperature inside the coil lags behind the temperature outside the coil such that the peak temperature is 2-3xc2x0 C. less. The time at peak temperature is also substantially less for the inner region than for the outer region. It follows that if the furnace profile is set to give the optimum heat treatment to the outer regions of the coil, the inner regions will be sub-optimal, and vice versa.
These results were obtained for a small coil of only a few mm thickness. It is a well known result of diffusion theory that thermal diffusion times scale as the square of the smallest dimension. Thus for a coil of 3xc3x97the thickness, the thermal diffusion time would be 9 times as long and the difference between inner and outer temperatures would be more than 20xc2x0 C.
In accordance with a first aspect of the present invention, we provide a method of heat treating a coil for use as a superconducting coil, the method comprising the steps of:
heating the coil in a furnace, the temperature of the furnace being controlled to perform a predetermined heating cycle; and,
passing a current through the coil for at least a portion of the heating cycle so as to further heat the coil by resistance heating.
The first aspect of the invention therefore provides a method of offsetting the problems of heat transfer through the coil by means of an electrical heating boost. With all the turns in the coil insulated from each other, the electric boost may be achieved by simply passing a current through the coil material.
Typically the predetermined heating cycle comprises:
a first heating portion in which the temperature of the coil is increased to a first predetermined temperature;
a second portion in which the temperature of the coil is maintained at the first predetermined temperature;
a third cooling portion in which the temperature of the coil is deceased to a second predetermined temperature;
a fourth cooling portion in which the temperature of the coil is deceased to a third predetermined temperature; and,
a fifth cooling portion in which the coil is unheated and allowed to cool.
However, it will be realized that other suitable heating cycles may be used depending on the type of coil material, the coil configuration and other factors.
When the abovementioned heating cycle is used, the method typically comprises passing the current through the coil during at least the first heating portion of the heating cycle. This ensures that the effects of poor heat transfer through the coil are minimized as the maximum temperature is reached.
Preferably the method further comprises controlling the current to maintain the inner surface of the coil at substantially the same temperature as the outer surface of the coil.
In this case, this may be achieved by controlling the current in accordance with the equation:       γ    ⁢          xe2x80x83        ⁢    C    ⁢          xe2x80x83        ⁢                  ⅆ        θ                    ⅆ        t              =            J      2        ⁢    ρ  
where: xcex3=mean density of the coil material
C=mean specific heat of the coil material             ⅆ      θ              ⅆ      t        =      required  rate  of  change  of  temperature  
J=mean current density in the coil material
xcfx81=mean resistivity of the coil material
Typically the predetermined heating cycle is performed in an atmosphere of pure oxygen or an oxygen/nitrogen mixture.
As far as the problems relating to oxygen diffusion are concerned, during the heat treatment cycle shown in FIG. 1, chemical changes occur in the superconducting material which cause oxygen to be evolved or absorbed. Accordingly, variations in the concentration of oxygen throughout the coil can cause variations in the effectiveness of the heat treatment.
A further effect results from the fact that the melting point of B2212 is reduced if the partial pressure of oxygen is reduced. Accordingly, for a tightly wound coil, there is thus a danger that during those times when oxygen is being absorbed, the partial pressure of oxygen in the innermost regions of the coil will be reduced. This reduction will lower the melting point of the B2212, which will produce two undesirable effects:
a) the maximum temperature will not be optimal for the best superconducting properties; and,
b) the B2212 will be more fully melted and will therefore be more likely to leak out of its silver sheath and cause shorted turns in the coil (a well known problem of melt processed coils).
In accordance with a second aspect of the present invention, we provide a coil for use as a superconducting coil, the coil being suitable for undergoing a heat treatment process in an oxidizing atmosphere, the coil comprising layers of insulating material interspersed between layers of the coil, the insulating material comprising a fibre mat arranged so as to allow diffusion of the oxidizing atmosphere throughout the coil.
The second aspect of the invention overcomes the problems associated with poor oxygen diffusion through the coil by constructing the coil to allow a free circulation of oxygen through the windings during the heat treatment. This circulation is achieved by making the insulation between each layer of the coil porous, so that oxygen may easily diffuse from the ends of the coil into the center.
Typically the fibre mat comprises a tissue paper layer with a number of spaced substantially parallel fibres glued thereon. The use of the parallel spaced fibers results in the formation of channels between the coil layers which helps enhance the flow of oxygen through the coil.
Preferably the coil is configured such that the fibres of the fibre mats are substantially aligned with the axis of the coil. This not only provides the abovementioned channels but also ensures the channels terminate at the ends of the coil. This allows the oxygen to flow into the ends of the channels so as to penetrate the coil, whilst ensuring that adjacent layers of the coil do not touch.
Typically the fibers are formed from a ceramic or a refractory oxide. Examples of suitable materials include aluminium or zirconium oxide, which are manufactured in the form of fibers, such as Nextel (copyright). However, any material that can withstand the high temperatures in an oxidizing atmosphere, may be used, although these must not react chemically with the coil material during reaction.
In accordance with a third aspect of the present invention, we provide a method of constructing a fibre mat for use in the above described coil, the method comprising:
forming a tube of tissue paper around a former;
fixing a single layer helix of fibre to the tissue paper tube; and,
cutting the tissue paper tube parallel to the tube axis to form the fibre mat.
This construction allows a fibre mat suitable for use in a superconducting coil to be easily and cheaply produced.
Typically the fibre is glued to the tissue paper, although other forms of fixing may be used.
It will be realized that the coil according to the second aspect of the invention may readily be heat treated using the method of the first aspect of the invention.
In this case, the predetermined heating cycle causes the tissue paper to burn away, which advantageously leaves the fibre to insulate the layers of the superconductor coil.
Typically, if the paper is to be burnt away, the first heating portion is modified to include a time period during which the temperature is maintained substantially constant to thereby ensure complete removal of the paper.
As will be appreciated, the invention makes improvements to the insulation technique and the heat treatment process which produce substantial improvements in the performance of resulting superconducting coils. Whilst it is primarily intended for use with superconducting coil materials made by the xe2x80x9cpowder in tubexe2x80x9d process from silver and the high temperature superconducting material Bismuth Strontium Calcium Copper oxide B2212, it could also be used with other superconducting materials requiring a reaction heat treatment.
Examples of the present invention will now be described with reference to the accompanying drawings, in which: